(1) Field of the Invention
The present invention relates generally to functional testing of circuit cards and more particularly to a self-contained functional test apparatus capable of performing functional tests on any one of a plurality of varied function, modular, digital logic circuit cards, including circuit cards employing transistor-transistor-logic (TTL) components.
(2) Description of the Prior Art
For years, testing of electrical systems has been required to provide indications of failed wiring and/or circuits. Prior art testing methods and apparatus range from time consuming manual operations to expensive and complex automatic testing devices that perform detailed testing. Accordingly, it is desirable to design testing devices that reduce the amount of manual operations while simultaneously minimizing the amount of detailed testing in order to minimize the cost and complexity of the device.
One such device is disclosed by Cheek et al in U.S. Pat. No. 3,728,616 issued Apr. 17, 1973. Cheek et al disclose a continuity testing apparatus that checks for wiring errors in connections between a plurality of pairs of terminals. The device uses a plurality of pairs of matched resistors whose relative values form a predetermined ratio. In addition, each pair of matched resistors have values that differ from the values of every other pair of resistors by some predetermined percentage. A known voltage is applied to each pair of resistors to produce a predetermined voltage drop across a selected one of each pair of resistors. If the actual voltage drop across the selected resistor differs from the predetermined voltage drop, a defective condition is indicated. Devices built according to the teachings of Cheek et al will only indicate breaks in wiring continuity by testing each and every wire connection. However, in many instances, it is more important to test a system functionally.
One such prior art functional tester is disclosed by Means in U.S. Pat. No. 4,251,765 issued Feb. 17, 1981. In particular, Means discloses a portable electrical system tester that may be connected to the "Caution" control system display panel of a military C-5A aircraft in order to detect failures in the display panel. However, the device is limited to the aforementioned specific use and is not readily adapted to any other use without a complete redesign.
Many of today's processing systems use a large number of specific function, modular circuit cards. The advantages achieved by using modular circuit cards in system processing design are numerous and generally well known in the art. For example, use of modular circuit cards allows particular processing functions to be assigned to a particular circuit card. Furthermore, since the costs associated with circuit cards and components has been greatly reduced over the years, it is now more cost effective to replace a failed circuit card than it is to troubleshoot the individual components thereon. Accordingly, it is becoming increasingly more important in these systems to develop testing equipment that can quickly provide an indication of a failed circuit card at the functional level.
Nowhere is this need more acute than in the military where the number of varied function modular circuit cards (called Standard Electronic Modules (SEM)) used in any given system can easily be 25 or more. For example, the Navy's SEM's used in torpedo systems are tested manually during the system's development stages. When these systems are placed "in service", testing of SEM's has heretofore often been done on complex automatic test equipment. However, the time critical nature of many of these military systems makes it imperative that the operator of the system be able to quickly detect functional level failures of any one of the varied function circuit cards. Furthermore, the use of transistor-transistor-logic (TTL) on these circuit cards requires that the system operator be able to juggle power supplies to apply the correct TTL logical high or low to multiple pins on the circuit cards in order to detect a failed circuit card. As noted above, this type of testing is slow, inaccurate and prone to operator error which often leads to operator induced failure. Furthermore, in many applications, including the aforementioned torpedo systems, power required for trouble-shooting a SEM card presents problems of safety to the technician.